Anode active material, method of preparing the same, and anode and lithium battery containing the material

ABSTRACT

An anode active material comprises metal core particles, metal nano wires formed on the metal core particles, pores between the metal core particles and the metal nano wires, and a carbon-based coating layer formed on a surface of the metal core particles and metal nano wires. In the anode active material according to the present invention, the metal core particles and metal nano wires are combined to form a single body, and a carbon-based coating layer is formed on the surface of the metal nano wires and metal core particles. Thus, volume changes in the pulverized metal core particles can be effectively buffered during charging and discharging, and the metal core particles are electrically connected through the metal nano wires. As a result, volume changes in the anode active material and degradation of the electrode can be prevented, thereby providing excellent initial charge/discharge efficiency and enhanced charge/discharge capacity.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 11/582,050, filed Oct. 16, 2006, which claims priority to andthe benefit of Korean Patent Application No. 10-2005-0097514, filed onOct. 17, 2005 in the Korean Intellectual Property Office, the entirecontents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to anode active materials, methods ofpreparing the same, and anode and lithium batteries containing the anodeactive materials. More particularly, the invention is directed to ananode active material having pores for buffering volume changes duringcharging and discharging. The invention is also directed to a lithiumbattery having a long cycle life, which lithium battery employs theanode active material.

2. Description of the Related Art

Non-aqueous electrolyte secondary batteries, which include anodescomprising lithium compounds, exhibit high voltage and high energydensity, and have therefore been widely researched. Lithium metal hasbeen studied as an anode material because of its high capacity. However,when metallic lithium is used as an anode material, lithium dendritesare deposited onto the surface of the metallic lithium during charging.The lithium dendrites reduce the charge/discharge efficiency of thebattery, and can cause short-circuits. Also, the risk of explosion andhigh sensitivity to heat and shock caused by the instability and highreactivity of metallic lithium has prevented the commercialization ofmetallic lithium anode batteries.

Carbon-based anodes have been used to address the problems of lithiumanodes described above. Lithium ions present in an electrolyte areintercalated and deintercalated between crystal facets of thecarbon-based anode, resulting in the occurrence of oxidation andreduction reactions. A battery including a carbon-based anode isreferred to as a rocking chair battery.

Carbon-based anodes have addressed various problems caused by lithiummetal, and have become popular. However, there is a need for lithiumsecondary batteries with high capacity in order to allow forminimization, reductions in weight and increases in power of portableelectronic devices. Lithium batteries containing carbon-based anodeshave low capacity due to the porous structure of carbon. For example,even for graphite (which is the carbon structure with the highestcrystallinity) the theoretical capacity of a LiC₆ composition is about372 mAh/g. This is less than 10% of the theoretical capacity of lithiummetal, which is about 3860 mAh/g. Therefore, despite the existingproblems with metallic lithium, much research has been activelyperformed to improve the capacity of batteries by introducing metalssuch as lithium into the anodes.

It is known that Li and alloys such as Li—Al, Li—Pb, Li—Sn and Li—Siprovide higher electrical capacities than carbon-based materials.However, when such alloys or metals are used by themselves, thedeposition of lithium dendrites occurs. Therefore, use of a suitablemixture of such alloys or metals and carbon-based materials has beenresearched to provide high electrical capacity while also avoidingproblems such as short circuits.

However, in such a mixture of metal materials and carbon-basedmaterials, the volume expansion coefficient during oxidation andreduction of the carbon-based materials is different from that of themetal materials, and the metal materials can react with the electrolyte.When charging the anode material, lithium ions are introduced into theanode material. When this happens, the anode expands and also becomesmore dense. On discharging, the lithium ions leave the anode, and thevolume of the anode decreases. At this time, if the anode contracts,there remain voids in the anode that are not electrically connected dueto the difference between the expansion coefficients of the carbon-basedmaterials and the metal materials. Due to the electrically insulatedvoids, the movement of electrons is not effective and the efficiency ofthe battery is decreased. Also, a reaction between the metal materialsand the electrolyte during the charging and discharging can decrease thelifetime of the electrolyte, thereby decreasing the lifetime andefficiency of the battery.

To overcome these problems, electrodes have been prepared by addingpolymer additives for providing elasticity and pores to the activematerial. The polymer additive is simply mixed with the active materialin the process of manufacturing the electrode, and provides elasticityand pores to the active material to enhance the cycle properties of thebattery. However, since the additive is not adhered to the activematerial, electrical insulation may occur when the elasticity of theadditive decreases due to long-term use.

An active material including two types of graphite having differentsurface densities has also been prepared. The energy density of theactive material is increased by reducing pores between active materialparticles by mixing ball-shaped particles and needle-shaped particles.Additionally, an anode active material having spherical graphite andplate graphite has been prepared. An electrolyte can easily impregnatesuch an anode active material, and thus, the electrical capacity of thebattery can be increased. However, only active materials, such asgraphite, that do not show large volume changes during charging anddischarging are suitable for such anode active materials. Thus activematerials that show large volume changes during charging and dischargingare not suitable.

Accordingly, a need exists for a more practical anode active materialhaving excellent charge and discharge properties, a long lifetime andhigh efficiency.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides an anode activematerial having a novel structure.

In another embodiment, the present invention provides an anode electrodeemploying the anode active material.

In yet another embodiment, the present invention provides a lithiumbattery having excellent initial charge/discharge efficiency andenhanced charge/discharge capacity by employing the anode electrode

In still another embodiment, the present invention provides a method ofpreparing the anode active material.

According to one embodiment of the present invention, an anode activematerial includes metal core particles, metal nano wires formed on themetal core particles as a single body, pores between the metal coreparticles and the metal nano wires, and a carbon-based coating layerformed on a surface of the metal core particles and the metal nanowires.

In one embodiment, the metal nano wires and the metal core particles inthe anode active material may include a metal that can be alloyed withlithium. Nonlimiting examples of suitable materials for the metal nanowires and the metal core particles include Si, Sn, Al, Ge, Pb, Bi, Sb,alloys thereof, and combinations thereof.

In one embodiment, the carbon-based coating layer may be a sinteredproduct of a polymer material. Nonlimiting examples of suitable polymermaterials include resins such as vinyl resins, phenol resins, celluloseresins, pitch resins, tar resins, and combinations thereof.

In another embodiment, the anode active material may further include acarbon-based active material. The carbon-based active material may be afibrous carbon-based active material.

According to another embodiment of the present invention, an anodeelectrode includes the anode active material.

According to yet another embodiment of the present invention, a lithiumbattery employs the anode electrode.

According to still another embodiment of the present invention, a methodof preparing an anode active material includes preparing a mixture bymixing a metal particle powder, a polymer material and a pore-formingmaterial, pulverizing the mixture, and sintering the pulverized mixture.The metal particle powder may include a metal that can be alloyed withlithium. Nonlimiting examples of suitable materials for the metalparticle powder include Si, Sn, Al, Ge, Pb, Bi, Sb, alloys thereof, andcombinations thereof.

Nonlimiting examples of suitable polymer materials include resins suchas vinyl resins, phenol resins, cellulose resins, pitch resins, tarresins, and combinations thereof.

Nonlimiting examples of suitable pore-forming materials include oxalicacid, citric acid, malic acid, glycine, ammonium carbonate, ammoniumbicarbonate, ammonium oxalate, and combinations thereof.

In one embodiment, pulverizing the mixture may be performed by highenergy milling, mechano fusion, or using a hammer mill.

In one embodiment, sintering the pulverized mixture may be performed ata temperature ranging from about 500 to about 1,400° C.

In one embodiment, the weight ratio of metal particle powder to polymermaterial for preparing the mixture ranges from about 1:50 to about 10:1.

In one embodiment, the metal particle powder may be silicon, the polymermaterial may be a polyvinyl alcohol, the pore-forming material may beoxalic acid, the pulverizing may be performed by high energy milling,and the sintering temperature may range from about 700 to about 1,000°C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by reference to the following detaileddescription when considered in conjunction with the attached drawings inwhich:

FIG. 1 is a scanning electron microscope (SEM) image of an anode activematerial prepared according to Example 3;

FIG. 2 is an SEM image of an anode active material prepared according toComparative Example 6;

FIG. 3 is a graphical representation of the results of charge/dischargetests performed on the lithium batteries prepared according to Example 3and Comparative Examples 5 and 6; and

FIG. 4 is a schematic perspective view of a lithium battery according toone embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to theaccompanying drawings, in which exemplary embodiments of the inventionare shown. The invention may, however, be embodied in many differentforms and should not be construed as limited to the embodiments setforth herein. Rather, the described embodiments are provided forillustrative purposes only.

In a conventional anode active material, only a carbon layer coated onthe surface of metal core particles buffers volume changes that occurduring charging and discharging. Such a technique does not adequatelyprevent battery degradation. In an anode active material having poresaccording to an embodiment of the present invention, the metal coreparticles and metal nano wires formed on the metal core particles arecombined in a single body, and a carbon-based coating layer is formed onthe surface of the metal nano wires and the metal core particles. Thus,volume changes in the pulverized metal core particles during chargingand discharging can be effectively buffered and the metal core particlesare electrically connected through the metal nano wires. As a result,volume changes in the anode active material can be prevented anddegradation of the electrode can be prevented, thus providing improvedinitial charge/discharge efficiency and enhanced charge/dischargecapacity.

As used herein, the term “metal nano wire” includes wire-shaped metalwith a nano-sized diameter having a large aspect ratio, regardless ofits manufacturing process. Accordingly, the metal nano wires may includenanorods, nanotubes, and the like.

An anode active material according to one embodiment of the presentinvention includes metal core particles; metal nano wires formed on themetal core particles, wherein the metal core particles and the metalnano wires form a single body; pores between the metal core particlesand the metal nano wires; and a carbon-based coating layer formed on thesurface of the metal core particles and the metal nano wires.

Metallic active materials conventionally used as anode active materialshave high capacity, but the charge/discharge properties of the batteriesare degraded due to large volume changes during charging anddischarging. Particularly, when a carbon layer is coated on the surfaceof the metal, the repeated charging and discharging disrupts the carboncoating layer, resulting in electrical insulation, and preventingreversible lithium ion charging and discharging. However, the anodeactive material according to one embodiment of the present invention isprepared by pulverizing the mixture of a polymer material, apore-forming material, and a metal active material by high energymilling, and sintering the pulverized mixture. According to thisembodiment, a newly exposed crystal surface (for example a (111) crystalsurface) of the pulverized metal active material (i.e. metal coreparticles) acts as a growth point of the nano wires. The metal nanowires connect the metal active materials while growing, and thecarbon-based coating layer (in which a polymer material is carbonized)is formed on at least a portion of the surface (or on the entiresurface) of the metal nano wires and the metal core particles. Thepore-forming material is vaporized during sintering to produce pores,and thus the anode active material has a porous structure.

FIG. 1 is a scanning electron microscope (SEM) image of an anode activematerial according to one embodiment of the present invention. Referringto FIG. 1, the metal core particles and the metal nano wires areirregularly and three-dimensionally arranged. Empty spaces exist betweenthe metal nano wires. The metal core particles formed on the surface ofthe metal nano wires have various and irregular shapes, and may bespherical, polygonal, or the like. Such various and irregular shapes maybe generated by breaking and deforming the metal core particles duringthe pulverizing and sintering processes. In contrast, in conventionalanode active materials, such as those illustrated in FIG. 2, metalactive materials are simply distributed and are not electricallyconnected due to the absence of metal nano wires.

In one embodiment, the metal nano wires and metal core particles mayinclude metals that can be alloyed with lithium. Nonlimiting examples ofsuitable metals for use as the metal nano wires and the metal coreparticles include Si, Sn, Al, Ge, Pb, Bi, Sb, alloys thereof, andcombinations thereof. However, it is understood that any metal commonlyused in lithium batteries can be used in the inventive anode activematerials.

In one embodiment, the carbon-based coating layer in the anode activematerial may be a sintered polymer material. Nonlimiting examples ofsuitable polymer materials include resins such as vinyl resins, phenolresins, cellulose resins, pitch resins, tar resins and combinationsthereof. However, it is understood that any polymer that can be sinteredto a carbon-based material by heating can be used as the polymermaterial.

In one embodiment, a fibrous carbon-based active material can beincluded in the anode active material. When such a fibrous carbon-basedactive material is used, more pores can be generated in the anode activematerial, thereby more effectively buffering volume changes in the metalcore particles.

In another embodiment, a carbon-based active material may be furtherincluded in the anode active material. The amounts of metal and carbonin the metal-carbon active material are not limited, and can beregulated as desired.

An anode according to one embodiment of the present invention includesan anode active material according to one of the embodiments describedabove. For example, the anode may be manufactured by mixing an anodeactive material and a binder to form an anode material composition andshaping the composition. The anode may also be manufactured by coatingthe anode material composition on a current collector such as a copperfoil.

In more detail, an anode material composition is first prepared. Theanode material composition may be directly coated on a current collectorsuch as copper foil to prepare an anode plate. Alternatively, the anodematerial composition may be cast on a separate support to form an anodeactive material film. The anode active material film is then removedfrom the support and laminated on a current collector such as copperfoil to prepare an anode plate. However, it is understood that themethod of manufacturing the anode is not limited to the above-describedembodiments.

Batteries require high current for charging and discharging to ensurehigh capacity. For this, battery electrodes must have low electricalresistance. To reduce the resistance of an electrode, various conductingagents are generally added. Carbon black, graphite microparticles, etc.are some nonlimiting examples of suitable such conducting agents.

A lithium battery according to one embodiment of the present inventionincludes an anode as described above. As shown in FIG. 4, the lithiumbattery 1 according to an embodiment of the present invention includesan electrode assembly comprising an anode 2 as described above, acathode 3 and a separator 4 positioned between the anode 2 and thecathode 3. The electrode assembly is contained within a battery case 5and sealed with a cap assembly 6. A lithium battery according to oneembodiment of the present invention can be manufactured as follows.

First, a cathode active material, a conducting agent, a binder, and asolvent are mixed to prepare a cathode active material composition. Thecathode active material composition is coated directly on a metalcurrent collector and dried to prepare a cathode plate. Alternatively,the cathode active material composition is cast on a separate support toform a film which is then separated from the support and laminated on ametal current collector to prepare a cathode plate.

The cathode active material may be any lithium-containing metal oxidecommonly known in the art. Nonlimiting examples of suitable such cathodeactive materials include LiCoO₂, LiMn_(x)O_(2x),LiNi_(x-1)Mn_(x)O_(2x)(x=1, 2), Ni_(1-x-y)Co_(x)Mn_(y)O₂(0≦x≦0.5,0≦y≦0.5), etc. In one embodiment, a compound capable of inducing theoxidation and reduction of lithium, such as LiMn₂O₄, LiCoO₂, LiNiO₂,LiFeO₂, V₂O₅, TiS or MoS, may be used as the cathode active material.

One nonlimiting example of a suitable conducting agent is carbon black.Nonlimiting examples of suitable binders for use in the cathode activematerial include vinylidenefluoride/hexafluoropropylene copolymers,polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate,polytetrafluoroethylene and mixtures thereof. Other nonlimiting examplesof suitable binders include styrene butadiene rubber polymers.Nonlimiting examples of suitable solvents include N-methylpyrrolidone,acetone, water, etc. The amounts of the cathode active material, theconducting agent, the binder, and the solvent are the same as thosecommonly used in lithium batteries.

Any separator commonly known in the lithium battery field may be used.In one embodiment, the separator may have low resistance to the transferof ions from an electrolyte and may allow the impregnation of theelectrolyte. Nonlimiting examples of suitable materials for theseparator include non-woven fabrics, woven fabrics, glass fiber,polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene(PTFE), and combinations thereof. In more detail, in a lithium ionbattery, a windable separator made of a material such as polyethylene orpolypropylene may be used. In a lithium ion polymer battery, a separatorthat allows the impregnation of an organic electrolyte solution may beused.

To manufacture the separator, a polymer resin, a filler, and a solventare mixed to prepare a separator composition. Then, the separatorcomposition is coated directly on an electrode and dried to form aseparator film. Alternatively, the separator composition is cast on aseparate support and dried to form a film. The film is then separatedfrom the separator and laminated on an electrode.

The polymer resin is not particularly limited, and may be any materialthat can be used as a binder for an electrode plate. Nonlimitingexamples of suitable polymer resins includevinylidenefluoride/hexafluoropropylene copolymers,polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, andmixtures thereof.

Nonlimiting examples of suitable solvents for use in the electrolyticsolution include propylene carbonate, ethylene carbonate, diethylcarbonate, ethyl methyl carbonate, methyl propyl carbonate, butylenecarbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, y-butyrolactone, dioxolane, 4-methyl dioxolane,N,N-dimethylformamide, dimethyl acetamide, dimethylsulfoxide, dioxane,1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene,nitrobenzene, dimethylcarbonate, methylethylcarbonate, diethylcarbonate,methylpropylcarbonate, methylisopropylcarbonate, ethylpropylcarbonate,dipropylcarbonate, dibutylcarbonate, diethyleneglycol, dimethyl ether,etc., and mixtures thereof.

Nonlimiting examples of suitable electrolytes include lithium salts suchas LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N,LiC₄F₉SO₃, LiSbF₆, LiAlO₄, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂)(wherein, x and y are naturalnumbers), LiCl, LiI, etc., and mixtures thereof.

A separator is positioned between the cathode plate and the anode plateto form a battery structure. The battery structure is wound or foldedand encased in a cylindrical or square battery case, and an organicelectrolyte solution is then injected into the cylindrical or squarebattery case to complete a lithium ion battery.

The battery structure may also be laminated to form a bi-cell structureand impregnated with an organic electrolyte solution. The resultantstructure is encased in a pouch and sealed to thereby complete a lithiumion polymer battery.

A method of preparing an anode active material according to oneembodiment of the present invention will now be described in moredetail.

In one embodiment, a method of preparing an anode active materialcomprises preparing a mixture by mixing a metal particle powder, apolymer material and a pore-forming material, pulverizing the mixture,and sintering the pulverized mixture. When the mixture is mechanicallypulverized, the size of the metal particles in the powder is reduced,the volume changes during charging and discharging are decreased, andthe pulverized metal particles, the polymer material and thepore-forming material are adequately mixed. When the pulverized mixtureis sintered, the pore-forming material is vaporized to generate emptyspaces within the mixture, the polymer material is carbonized to form aconductive carbon-based coating layer, and the metal nano wires growfrom the metal particles that contacted the carbon-based coating layer.As a result, an anode active material according to one embodiment of thepresent invention is obtained.

In one embodiment, the metal particle powder may include a metal thatcan be alloyed with lithium. Nonlimiting examples of suitable metals forthe metal particle powder include Si, Sn, Al, Ge, Pb, Bi, Sb, alloysthereof and mixtures thereof. However, any metal commonly used inlithium batteries can be used.

Nonlimiting examples of suitable polymer materials include resins suchas vinyl resins, phenol resins, cellulose resins, pitch resins, tarresins and mixtures thereof. However, any polymer that can be sinteredto a carbon-based material by heating can be used as the polymermaterial.

Nonlimiting examples of suitable pore-forming materials include oxalicacid, citric acid, malic acid, glycine, ammonium carbonate, ammoniumbicarbonate, ammonium oxalate and mixtures thereof. However, anymaterial that can be vaporized by heating and that produces pores in theactive material can be used.

Nonlimiting examples of suitable pulverizing methods include high energymilling, mechano fusion or using a hammer mill. However, any method thatpulverizes the metal particle powder into smaller-sized particles touniformly mix the polymer material and the metal particle powder can beused.

In one embodiment, sintering of the pulverized mixture is performed at atemperature ranging from about 500 to about 1,400° C. In anotherembodiment, the sintering is performed at a temperature ranging fromabout 700 to about 1,000° C. When the sintering temperature is greaterthan about 1,400° C., the metal core particles melt and the shapes ofthe metal particles change.

In one embodiment, the polymer material is carbonized at a temperatureof about 500° C. When the temperature is lower than about 500° C., theability to prevent volume changes is too low.

In one embodiment, the weight ratio of the metal particle powder to thepolymer material ranges from about 1:50 to about 10:1. In anotherembodiment, the weight ratio of the metal particle powder to the polymermaterial ranges from about 1:10 to about 5:1. When the weight ratio isless than about 1:50, the capacity per unit weight of the activematerial is low. When the weight ratio is greater than about 10:1, theability of the carbon-based coating layer to prevent the volume changeis too low.

In an embodiment of the present invention, the metal particle powder maybe silicon, the polymer material may be polyvinyl alcohol, thepore-forming material may be oxalic acid, the pulverizing method may behigh energy milling, and the sintering temperature may range from about700 to about 1,000° C.

Hereinafter, the present invention will be described in more detail withreference to the following examples. These examples are provided forillustrative purposes only and are not intended to limit the scope ofthe invention.

Preparation of Anode Active Materials

EXAMPLE 1

1 g of silicon metal powder having an average diameter of less than 43μm, 1 g of polyvinyl alcohol (PVA) powder with a molecular weight of500, and 3 g of oxalic acid were mixed to form a mixture. The mixturewas pulverized by high energy mechanical milling using a SPEX CertiPrep8000M mill. The pulverized mixture was heated under an argon atmosphereto 800° C. for 10 hours to completely carbonize the PVA. Then, thecarbonized product was pulverized in a mortar to prepare an anode activematerial.

COMPARATIVE EXAMPLE 1

1 g of silicon metal powder having an average diameter of 43 μm was usedas an anode active material.

COMPARATIVE EXAMPLE 2

1 g of silicon metal powder having an average diameter of 43 μm and 1 gof polyvinyl alcohol (PVA) powder with a molecular weight of 500 wereadded to 10 ml of distilled water and stirred until the PVA wascompletely dissolved. Then, the mixture was gradually heated whilestirring until the water was completely evaporated to thereby obtain asolid containing a mixture composed of the above two components. Thesolid was heated under an argon atmosphere to 800° C. for 10 hours tocompletely carbonize the PVA. Then, the carbonized product waspulverized in a mortar to prepare an anode active material.

Manufacture of Anode Electrodes

EXAMPLE 2

1 g of the anode active material powder prepared in Example 1, 8.6 g ofgraphite powder with an average diameter of 10 μm, 2 g of 10 wt %styrene butadiene rubber (SBR), and 0.2 g of carboxymethyl cellulose(CMC) were mixed, and 20 mL of distilled water was added to the mixture.Then, the reaction mixture was stirred for 30 minutes using a mechanicalstirrer to prepare a slurry.

The slurry was coated to a thickness of about 200 μm on a copper (Cu)current collector with a doctor blade and then dried. The resultantstructure was again dried in a vacuum at 110° C. to manufacture an anodeplate.

COMPARATIVE EXAMPLE 3

An anode electrode was manufactured as in Example 2, except that 1 g ofthe silicon metal powder prepared in Comparative Example 1 was usedinstead of the active material powder prepared in Example 1.

COMPARATIVE EXAMPLE 4

An anode electrode was manufactured as in Example 2, except that 1 g ofthe anode active material prepared in Comparative Example 2 was usedinstead of the anode active material powder prepared in Example 1.

Manufacture of Lithium Batteries

EXAMPLE 3

A 2015 standard coin cell was manufactured using the anode platemanufactured in Example 2, a counter electrode made of lithium metal, aPTFE separator, and an electrolyte solution including 1M LiPF₆ dissolvedin a mixed solvent of ethylene carbonate (EC) and diethyl carbonate(DEC) in a volume ratio of 3:7.

COMPARATIVE EXAMPLE 5

A lithium battery was manufactured as in Example 3, except that theanode plate prepared in Comparative Example 3 was used.

COMPARATIVE EXAMPLE 6

A lithium battery was manufactured as in Example 3, except that theanode plate prepared in Comparative Example 4 was used.

Charge/Discharge Experiments

The lithium batteries manufactured in Example 3 and Comparative Examples5 and 6 were charged and discharged at 0.1 C. The results are shown inFIG. 3. Initial charge/discharge capacity and initial charge/dischargeefficiency are shown in Table 1.

TABLE 1 Charge Discharge Initial charge/ capacity capacity discharge(mAh/g) (mAh/g) efficiency (%) Example 3 802 651 81 Comparative 750 45160 Example 5 Comparative 680 506 74 Example 6

As shown in Table 1 and FIG. 3, the initial charge/discharge efficiencywas greater than 80% in the lithium battery manufactured in Example 3,but the initial charge/discharge efficiency was less than 75% in thelithium batteries manufactured in Comparative Examples 5 and 6. Thecapacity retention of the lithium battery manufactured in Example 3 wasgreater than 50%, which is similar to that of Comparative Example 6, butthe capacity retention of the lithium battery manufactured inComparative Example 5 was less than 40%. Although the capacity retentionof the lithium battery manufactured in Example 3 was similar to that ofComparative Example 6, the discharge capacity was 20% higher after 30cycles in the lithium battery manufactured in Example 3 than in thelithium battery manufactured in Comparative Example 6.

The metal core particles and the metal nano wires beingthree-dimensionally arranged with pores provides relatively widesurfaces without electrical disconnection. This enables most of themetal active materials to be used reversibly during charging anddischarging of the lithium ions, thereby increasing the initialcharge/discharge efficiency and the initial charge/discharge capacity,as shown by the results of the test performed on the battery of Example3.

The pores between the metal core particles and the metal nano wires ofthe anode active material effectively buffer the volume changes in themetal active materials during charging and discharging, prevent volumechanges in the anode active material, prevent cracks in the anode activematerial, and maintain electrical conductivity. As a result, the lithiumbattery of Example 3 exhibits enhanced discharge capacity after 30cycles compared to the lithium batteries of Comparative Examples 5 and 6(which have capacity retention rates similar to that of the lithiumbattery in Example 3).

In the anode active material with pores according to the presentinvention, the metal core particles and metal nano wires formed on themetal core particles are combined into a single body, and a carbon-basedcoating layer is formed on the surface of the metal nano wires and themetal core particles. Thus, the volume changes in the pulverized metalcore particles can be effectively buffered during charging anddischarging, and the metal core particles are electrically connectedthrough the metal nano wires. As a result, volume changes in the anodeactive material can be prevented and degradation of the electrode can beprevented to provide excellent initial charge/discharge efficiency andenhanced charge/discharge capacity.

While certain exemplary embodiments of the present invention have beenillustrated and described, it will be understood by those of ordinaryskill in the art that various changes and modifications may be made tothe described embodiments without departing from the spirit and scope ofthe present invention as defined by the attached claims.

What is claimed is:
 1. A method of preparing an anode active materialfor a lithium battery, the method comprising: mixing a metal particlepowder, a polymer material and a pore-forming material to prepare amixture; pulverizing the mixture; and sintering the pulverized mixtureto prepare an anode active material comprising a carbonized polymermaterial, wherein the metal particle powder comprises a metal selectedfrom the group consisting of Si, Sn, Al, Ge, Pb, Bi, Sb, alloys thereof,and combinations thereof as a main component.
 2. The method of claim 1,wherein the polymer material comprises a resin selected from the groupconsisting of vinyl resins, phenol resins, cellulose resins, pitchresins, tar resins and combinations thereof.
 3. The method of claim 1,wherein the pore-forming material comprises a compound selected from thegroup consisting of oxalic acid, citric acid, malic acid, glycine,ammonium carbonate, ammonium bicarbonate, ammonium oxalate andcombinations thereof.
 4. The method of claim 1, wherein pulverizing themixture comprises a method selected from the group consisting of highenergy milling, mechano fusion and using a hammer mill.
 5. The method ofclaim 1, wherein sintering the pulverized mixture is performed at atemperature ranging from about 500 to about 1,400° C.
 6. The method ofclaim 1, wherein sintering the pulverized mixture is performed at atemperature ranging from about 700 to about 1,000° C.
 7. The method ofclaim 1, wherein the weight ratio of the metal particle powder to thepolymer material ranges from about 1:50 to about 10:1.
 8. The method ofclaim 1, wherein the weight ratio of the metal particle powder to thepolymer material ranges from about 1:10 to about 5:1.
 9. The method ofclaim 1, wherein the metal particle powder comprises silicon, thepolymer material comprises polyvinyl alcohol, the pore-forming materialcomprises oxalic acid, the pulverizing the mixture comprises high energymilling, and the sintering the pulverized mixture is performed at atemperature ranging from about 700 to about 1,00° C.
 10. The method ofclaim 1, wherein in the sintering the pulverized mixture, metal nanowires grow from the metal particle powder.